Electrocoat, or electrodeposition coating, methods have been used commercially for applying decorative and protective coatings to metallic substrates for a number of years. In the electrodeposition coating process, a conductive article or substrate that is to be coated is used as one electrode in an electrochemical cell. The article is submerged in an aqueous dispersion of the coating composition, which contains a charged, preferably a cationic, resin. The resin is deposited onto the article by applying an electrical potential between the article and a second electrode (which may be, for example, the walls of the vessel holding the bath). The coating deposits onto the article until it forms an insulating layer on the article that essentially prevents more current from being passed. The electrocoating process is particularly suited to applying a continuous and uniform protective primer layer to an article or workpiece that has complex shape or construction. When the surfaces of the article closest to the other electrode have been coated and insulated, the current then deposits the coating onto recessed areas and other less accessible areas until an insulating coating layer is formed on all conductive surfaces of the article or workpiece, regardless of how irregularly shaped the article.
Electrocoat processes, particularly for coating automotive bodies and parts, usually employ a thermosetting coating composition comprising an ionic, preferably a cationic, principal resin and a polyfunctional oligomeric or monomeric crosslinking agent that is capable of reacting with the principal resin to cure or crosslink the coating. The crosslinking agent is associated with the principal resin in the dispersion and is deposited along with the principal resin onto the article or workpiece. After deposition, the deposited coating may be cured to a crosslinked, durable coating layer.
A number of crosslinking mechanisms may be employed. One curing mechanism utilizes a melamine formaldehyde resin curing agent in the electrodepositable coating composition to react with hydroxyl groups on the electrodeposited resin. This curing method provides good cure at relatively low temperatures (e.g., 132.degree. C.), but the crosslink bonds contain undesirable ether linkages and the resulting coatings provide poor overall corrosion resistance as well as poor chip and cyclic corrosion resistance. In an alternative curing method, polyisocyanate crosslinkers may be reacted with hydroxyl groups on the electrodeposited resin. This curing method provides desirable urethane crosslink bonds, but it also entails several disadvantages. In order to prevent premature gelation of the electrodepositable coating composition, the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime or alcohol). Blocked polyisocyanates, however, require high temperatures (e.g., 150.degree. C. or more) to unblock and begin the curing reaction. The resulting electrocoats can also be susceptible to yellowing. Moreover, the release of the volatile blocking agents during cure increases emissions and decreases the amount the solid material in the coating composition that ultimately becomes part of the cured film formed on the substrate.
There is thus a need in the art for electrodepositable coating compositions that could provide desirable urethane crosslink linkages, but avoid the problems that accompany the use of blocked polyisocyanate curing agents.
U.S. Pat. No. 5,431,791 describes a cathodic electrodeposition method that applies a coating layer of a resin having a plurality of acid-salted primary amine groups and a curing agent having a plurality of cyclic carbonate groups. In this method, high levels of the salted primary amine were needed in order to achieve desirable levels of crosslinking. The high content of salted primary amine, however, can cause excessive bath conductivity.